1. Field of the Invention
This invention relates to airframe design. More particularly, this invention relates to solid fuel rocket motors. By way of further characterization, the invention relates to a solid fuel rocket motor which has improved immunity to destructive fires during storage prior to firing. Still more particularly, the invention will be described as it pertains to rocket motors having resistance to fuel fires external thereto.
2. Description of the Prior Art
Navy carrier operations provide the potential for aircraft fuel fires to occur in the vicinity of weapons and ordnance. Many in-fleet rocket motors react after a brief exposure to enveloping fuel fire flames. The severity of the reaction varies from mild burning, which is acceptable, to violent case rupture.
Past attempts to solve the problem have included placing a thermal barrier on the exterior of the rocket motor casing or warhead in an attempt to thermally insulate the rocket motor propellant or explosive material from fire. This approach increases the length of time the ordnance item may be exposed to a fire without exploding by keeping the internal temperature low. If the fire is not extinguished within a short period of time, the internal temperature will increase and the ordnance item will ignite and explode. Explosion and violent ignition of a heated motor may occur when the propellant grain is ignited along the central void in the grain as is the case during normal motor operation. If combustion can be started from the outside of the grain and properly vented, the severity of the reaction may be lessened.
Thus, a technique of providing a small vent/hole, before propellant ignition occurs over a significant surface area, is required. It has been observed that when ignition commences in such a fashion that a large area vent is produced by enlargement of the small hole due to "torching" of exiting propellant gases. The combustion is mild and controllable. Thus, if the rupture can be induced to occur at a desired location and early in the exposure to the fuel fire, a margin of safety is achieved.